Alongside magnetic resonance tomography (MRI) positron emission tomography (PET) has also become increasingly widespread of recent years in medical diagnosis. While MRI is an imaging method for displaying structures and slice images in the interior of the body, PET enables a visualization and quantification of metabolic activities in vivo.
PET uses the particular properties of positron emitters and positron annihilation in order to determine the function of organs or cell areas quantitively. In this case, before the examination the patient is administered appropriate radiopharmaceuticals that are marked with radionuclides. In the event of decay, the radionuclides emit positrons that interact with an electron after a short distance, resulting a so-called annihilation. Two gamma quanta are produced in this case and fly apart from one another in opposite directions (offset by 180°). The gamma quanta are detected by two opposite PET detector modules inside a specific time window (coincidence measurement), as a result of which the location of the annihilation is determined at a position on the connecting line between these two detector modules.
For detection, in the case of PET the detector module must generally cover a major part of the length of the gantry arc. The module is subdivided into detector elements with a side length of a few millimeters. When detecting a gamma quantum, each detector element generates an event record that specifies the time and the detection location, that is to say the appropriate detector element. These items of information are transferred to a fast logic unit and compared. If two events coincide with a maximum time spacing, it is assumed there is a gamma decay process on the connecting line between the two associated detector elements. The reconstruction of the PET image is performed with the aid of a tomography algorithm, that is to say the so-called back projection.
A superposed imaging of the two methods is desirable in many instances on the basis of the different items of information that are obtained by MRI and PET.
For future systems, an attempt is currently being made to combine the MRI and PET imaging methods in one unit, and to make it possible to use them simultaneously, as far as possible. An avalanche photodiode array (APD photodiode array) with an upstream array of lutetium oxyorthosilicate crystals (LSO crystals) is favored in this case as a PET detector module. FIG. 5 shows a schematic of a perspective view of the structure of a conventional PET detection module in accordance with the prior art. The PET detector module comprises an APD photodiode array 5 with an upstream array of LSO crystals 4. An electrical amplifier circuit (AMP) 6 is arranged in the axial direction of the PET detection module, that is to say downstream of the APD photodiode array 5.
Because of the crystals, the PET detector module is a relatively expensive component. Cost effective approaches would be advantageous for mass production.
Another proposal uses a gantry in whose z-direction (longitudinal direction) and circumferential direction an extended array of APD/LSO detectors is arranged. Seated in a base arrangement inside the PET detector is an MRI antenna that is separated from the detector for the purpose of mutual interference decoupling by a PET-transparent, MRI-compatible RF shield.
An MRI unit usually has a field of view (FOV) approximately 50 cm long in the longitudinal direction (z-direction). A PET system usually has a field of view (FOV) only approximately 15 cm long in the z-direction. In order to reduce costs for mass production, it is possible to utilize the fact that the MRI examination generally lasts much longer than the PET data acquisition. It is thereby possible to take PET pictures sequentially over time by displacing the detector module. When the MRI unit has a field of view approximately 50 cm long in the z-direction, and the PET system has a field of view approximately 15 cm long in the z-direction, it would then be necessary to displace the PET system 3 to 4 times in order to cover the field of view of the MRI unit. The disadvantage of this method is that there is a need for mechanical movement.
WO 2006/071922 A2 discloses a device for superposed magnetic resonance tomography and positron emission tomography imaging that exhibits a magnetic resonance tomography tube and a multiplicity of positron emission tomography detection units arranged opposite one another in pairs about the longitudinal direction of the magnetic resonance tomography tube.